1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and its manufacturing method. In particular, the present invention relates to a structure of a substrate for display device of the LCD device and its manufacturing method.
2. Description of the Prior Art
In recent years, an active-matrix type LCD device is widely used as a display with high resolution. Such LCD device has a liquid crystal sandwiched between an active matrix substrate as a first substrate and an opposite substrate as a second substrate. A typical example of the active matrix substrate is provided with switching elements such as thin film transistors (TFTs) at each of plurality of pixels and the opposite substrate is provided with color filters and a black matrix. In this LCD device, orientation directions of liquid crystal molecules are changed by an electric field between electrodes provided on the active matrix substrate or TFT substrate and the opposite substrate, respectively or between a plurality of electrodes provided on the TFT substrate and then an amount of light transmittance is controlled at each pixel.
A conventional structure of the TFT substrate will be described with reference to FIG. 1 and FIGS. 2A to 2C. A gate wiring 102 and a drain wiring 106 perpendicular to the gate wiring 102 via a gate insulating film 103 are formed on an insulating substrate 101 made of, for example, a glass substrate. A TFT is provided in a vicinity of a crossing portion between the gate wiring 102 and the drain wiring 106. As shown in FIG. 2A, the TFT is provided with a gate electrode 102A formed on the insulating substrate 101 and the gate insulating film 103 formed on the gate electrode 102A. A lamination film of an amorphous silicon film (described as an a-Si film 104) and an n+ type amorphous silicon film (described as an n+ a-Si film 105) is formed on the gate insulating film 103. A drain electrode 106A and a source electrode 107 both made of the same material as that of the drain wiring 106 are formed on the n+ a-Si film 105 to configure the TFT. A passivation film 108 formed of an SiN film or the like is formed so as to cover the TFT. Between the drain wiring 106 and a pixel electrode 110, agate light-shielding film 102B is formed in the same layer as the gate wiring 102 as shown in FIG. 1. The gate light-shielding film 102B shields the light incident on the periphery of the pixel electrode 110. The source electrode 107 is connected to the pixel electrode 110 via a contact hole 109 on the TFT substrate of FIG. 1.
The larger the LCD device becomes the longer wirings such as the gate wiring and the drain wiring become and the resistance of overall wirings increases. As a result, signal delays occur and display qualities becomes prone to deterioration. Moreover, in order to respond to demands for much higher density and improvements in aperture ratio of the LCD device, it is necessary to form wirings narrower. However, when wirings become narrow, the resistance increases similarly to the case described before and the display qualities deteriorate due to the signal delays. Since aluminum (Al), which has hitherto been used as a material for the gate wirings, has an especially high resistance, it is impossible to obtain an LCD device with a high image quality due to a signal delay at the gate wiring. Accordingly, a method using copper (Cu) whose resistance is lower than that of Al as a material for the wirings in order to reduce the resistance of wirings is proposed in Japanese Patent Laid-open Official Gazette No. 2002-202519 and the like.
However, due to mutual reactions between Cu and oxide silicon (SiO2), silicon nitride (SiN), or the like in Si-containing films, film qualities tend to deteriorate when Cu is used as the material for wirings. Moreover, because of poor resistances to chemicals and corrosion, Cu is likely to be etched during manufacturing steps of the TFT substrate. Furthermore, there is a problem where Cu cannot be used as a single layer due to a drawback of Cu such as its poor adhesion to substrates.
In addition, the gate electrode 102A, the gate light-shielding film 102B, and the gate wiring 102 are formed on the insulating substrate 101 made of a glass substrate and so forth on the substrate for display of the conventional LCD device as shown in FIGS. 2A to 2C. For this reason, influences of step heights of wirings in a lower layer side appear on an upper layer side and in a liquid crystal layer. When a film thickness of the wirings is made larger in order to reduce the resistance of the wirings, the influence of difference in level of the wirings in the lower layer side will become obvious. As a result, there was a problem that disclination defects due to disorder of liquid crystal orientation and disconnection failures of an upper layer wiring tend to occur readily.